1. Field of the Invention
The invention relates to switchmode power converters. In particular, the invention relates to switchmode power converters in which characteristics and properties of the fundamental buck or boost converter forms are advantageously integrated. More particularly, the invention relates to the advantageous integration of the previously integrated buck-boost converter form with the fundamental buck converter form.
2. Discussion of Prior Art
Three basic circuit families are commonly used in switchmode power supplies; namely, buck, boost, and buck-boost (commonly known as "flyback"). The buck-boost (flyback) converter is probably the earliest example of advantageous integration of the fundamental buck and boost converter forms. Although some authority treats the buck-boost converter as a fundamental form, another body of opinion holds that the buck-boost converter is an integrated form.
Whatever the genesis of the buck-boost converter form, there is consensus of authority regarding a topological property of the continuous mode control-to-output transfer function of both the buck-boost converter form, M=D/(1-D), and the boost converter form, M=1/(1-D). This consensus holds that there is present in ALL continuous mode, PWM controlled, fixed frequency, switch mode boost and buck-boost converter forms a non-minimumphase property, the right half s-plane zero. Dubbed the RHP zero, this first-order property is attributable to the inductor current/load current discontinuity consequential to the (1-D) term. This RHP zero never occurs in circuits of the buck family. Further consensus holds that this RHP zero is a disadvantageous property, as manifested in either the fundamental boost converter form or the integrated buck-boost converter form, since either stability or dynamic response is severely compromised. This RHP zero has been addressed with wry humor "A right-half-plane zero is included in every boost-derived converter transfer function at no extra cost.", a measure of vituperation ". . . very nasty right half plane zero . . . ", and a sort of despair ". . . impossible to compensate. The non-existence of a buck-boost (flyback) converter form equivalent to the full-wave buck (forward) converter form has been asserted. Diverse efforts to mitigate this RHP zero include discontinuous mode operation, leading edge modulation, power and control component value manipulation, and average current mode control techniques. Contemporary converter control loop compensation teaching extensively addresses the need for effective containment of this RHP zero effect.
Postulated in 1967, and analytically confirmed in 1972, this RHP zero effect has been the subject of continuous and intense scrutiny for more than two decades. The justification for this effort is found in the collective of advantageous properties of the buck-boost converter form, among which are simplicity, ease of multiple-output implementation, the compound voltage transfer function, wide input line range, reduced semiconductor stress, load protection, and the same power densities and efficiencies as forward (buck) converters.
This RHP zero effect notwithstanding, the buck-boost converter form (flyback) has found widespread usage in power conversion applications, with proprietary work in both single-ended and full-wave configurations as well as extensive public domain effort.
The foregoing discussion, along with well established application criteria, makes it abundantly clear that the already very popular buck-boost converter form would find substantially greater utility if this RHP zero effect were eliminated.